Axial swashplate type piston pumps, such as the one illustrated in prior art FIG. 1, are the main components in most hydrostatic transmissions. Such pumps generate a pump action by causing each piston to reciprocate within its piston bore with reciprocation of the piston being caused by a swashplate that the pistons impinge upon as a cylinder block rotates. Pump displacement, of course, depends on the bore size and the piston stroke as well as the number of pistons that are utilized. The swashplate pivot angle determines the length of the piston stroke, which can be varied by changing the noted swashplate angle, as is well known in the art. For a variable displacement pump, the swashplate can be integrated on a pivoted yoke, as illustrated in prior art FIGS. 2 to 4, near the center of the pump, or mounted onto cradle surfaces of either the front housing, as illustrated in prior art FIG. 5, or at the end cap, again as is well known in the art.
With the swashplate rotating at its extreme pivot angle, a maximum fluid volume is discharged from the pump outlet. However, when the swashplate is centered with the cylinder barrel, the pump will not generate any fluid flow. In some axial swashplate type piston pump designs, the swashplates have the capability of crossing over-center which results in the increasing and decreasing of the fluid flow volumes being generated at opposite ports. In an over-center axial swashplate piston pump, each system port can be either an inlet or an outlet port depending on the pivot angle of the swashplate. Over-center axial swashplate piston pumps are very widely used in hydrostatic transmissions and are generally referred to as closed-loop pumps.
Different control options are available for most variable displacement axial piston pumps for dynamically controlling the pivot angle of the swashplate according to the special requirements of the specific application. For example, constant pressure control, constant horsepower control, and electrical flow control are typical popular options that are used to control the dynamic response of the swashplate of such a pump. These noted control options are typically integrated into a control device that is attached to the pump housing. This control device is generally known as a compensator and controls the angular position of the pump swashplate via a servo-piston, examples of which are illustrated in prior art FIGS. 2 to 5. Such compensators are very widely used in most medium and heavy-duty axial piston pumps. Light duty axial piston pumps, which typically use spherically-nosed pistons instead of using pistons with slippers for low cost production, normally have no other control options beyond direct mechanical control. In other words, an external lever is coupled to the trunnion shaft of the axial piston pump for changing the pump fluid flow rate and direction during the operation thereof. The trunnion shaft is linked to the swashplate, in a manner well known in the art, with the rotation or pivoting thereof causing an angular change of the swashplate. The axial piston pump illustrated in prior art FIG. 1 includes a return-to-neutral device wherein a large plate having two spaced holes is connected to the pump trunnion shaft. In order to stroke this pump, a separate linkage (not shown) is attached to one of the noted holes.
The patent literature includes a large number of references pertaining to axial swashplate type variable displacement piston pumps that include control devices therefor. Examples thereof, which will be briefly discussed hereinafter, include: U.S. Pat. No. 3,677,362 to Chatterjea; U.S. Pat. No. RE 31711 to Horiuchi; and U.S. Pat. No. 6,443,706 B1 to Deininger et al.
Turning first to U.S. Pat. No. 3,677,362 to Chatterjea, the structure thereof pertains to a directional valve with a two-axis mechanical handle/joystick for controlling a hydrostatic transmission but no details are provided about how to attach the actuator to the pump swashplate. One embodiment of the control devices of the present invention utilizes a directional flow control valve with a single-axis handle, while other embodiments utilize electronic joystick control for two-axis applications.
U.S. Pat. No. RE 31711 to Horiuchi sets forth a conventional structure to control the angle of the swashplate by using an axial servo spool/piston at the uppermost location of the swashplate. In the control devices of the present invention, an external actuator is utilized in an operative interconnection with the trunnion shaft at that location.
U.S. Pat. No. 6,443,706 B1 is similar to above-noted U.S. Pat. No. RE 31711 to Horiuchi and uses a servo-spool/piston at both the uppermost and lowest ends of the swashplate in order to control the swashplate angle, rather than at the trunnion shaft location as is the case with the control devices of the present invention. In addition, a rotary valve is used as a directional flow control valve to supply fluid flow into and out of the servo spools. In contrast thereto, the control devices of the present invention utilize a rotary actuator, not a rotary directional flow control valve, to rotate the swashplate via the trunnion shaft, and not via a servo spool.
In all of the prior art structures, the actuators and/or cylinders for pivoting the pump swashplates are physically located inside the pumps themselves and are controlled by compensators. Thus, there is direct control of the swashplate within the pump itself which is expensive and complex. In addition, servicing and maintenance require that the pump unit be opened up and at least partially disassembled.
It should be understood that for most variable displacement pumps, an optional, complicated, servo control system is the preferred control system for differing hydraulic systems. Such control systems include the previously-noted constant pressure control, constant horsepower control and electronic control. Such a servo system usually provides for an interaction between the work condition, i.e., the work being performed, and the pump swashplate. For that reason, the necessary servo valve is generally placed inside the hydraulic pump itself, or attached to the pump at a location remote from the swashplate trunnion shaft since the servo valve requires its flow passages to be connected to the pump end cover.
For zero-turn-radius (ZTR) machines or vehicles, no servo valve is required since the human operator can sense the working conditions and physically make the necessary changes based upon his/her decision. Thus, the simplest and most logical way to add automatic swashplate control is via a trunnion shaft location thereof in the manner set forth in the present invention.